ADAC Test Finds Slower Home EV Charging Wastes More Electricity

An electric vehicle consumes power before any of that energy reaches its battery. The battery-management system, onboard charger and supporting 12-volt electronics remain active throughout a charging session. When charging takes longer, those background loads keep running longer and consume a larger fraction of the electricity purchased from the wall.

That mechanism helps explain the result of an ADAC comparison involving a Mercedes CLA, Renault 5, Tesla Model Y, Volvo EX30 and Volkswagen ID.7. All five became less efficient as home AC charging power was reduced. The Mercedes produced the most extreme result, losing 24.2% of the wall energy while charging from household power.

The German automobile association compared roughly 2.3-kilowatt household charging, an approximately 4-kilowatt reduced wall-charger setting and an 11-kilowatt wall charger. The household socket was the least efficient option for every vehicle. At 11 kilowatts, all five recorded losses of approximately 5% to 7%.

Why a smaller load can produce a larger loss

EV battery and charging engineer Shayan_EV described the charging system’s housekeeping demand as roughly 100 to 300 watts. Unlike the power entering the battery, much of that supporting load does not shrink in proportion to the charger setting.

Using a 200-watt background load as an illustration, the electronics would consume about 14% of the input from a 1.4-kilowatt North American household outlet. At 10 kilowatts, the same 200 watts would represent only around 2% of incoming power. Conversion losses, resistance, battery conditioning and temperature can also affect the wall to battery result, but fixed overhead explains why stretching a session can be counterproductive.

The comparison concerns AC home charging, where the vehicle’s onboard charger converts alternating current into the direct current stored by the battery. It should not be conflated with high-power public DC fast charging, which bypasses that onboard conversion stage and operates at a substantially different power level.

Shayan noted that a 40-amp Level 2 home charger remains mild compared with DC fast charging that can exceed 150 kilowatts. That boundary is important because evidence about battery stress at very high DC power does not show that deliberately restricting current on a properly installed Level 2 system provides the same battery-health benefit.

A Geotab analysis of more than 22,700 electric vehicles found that vehicles frequently using high-power DC fast charging above 100 kilowatts experienced greater average degradation than vehicles relying mainly on AC or lower-power charging. The analysis addresses the large gap between ordinary home charging and high-power DC charging; it does not demonstrate an advantage from reducing one approved Level 2 setting to another.

The extra electricity can become an ownership cost

To estimate the financial effect, Shayan modeled 9,300 miles of annual driving, consumption of 29 kilowatt-hours per 100 miles and electricity priced at 18 cents per kilowatt-hour. Under those assumptions, household-socket charging instead of 11-kilowatt charging would add approximately $40 to $50 a year for most of the tested vehicles. The Mercedes result raised the modeled difference to about $120 annually.

Those figures are scenarios rather than universal bills. Electricity prices, vehicle efficiency, climate and annual mileage vary. Utility tariffs can also change the decision: a lower charger setting may still cost less if it keeps a household below a demand threshold or allows charging during a cheaper rate period, even while consuming more total energy.

Electrical limits take priority as well. Reduced current remains appropriate when required by older wiring, circuit capacity or an electrician’s configured limit. Drivers should not raise charging current beyond the approved capacity of the circuit, wiring and charging equipment.

For a properly installed Level 2 system operating within those limits, Shayan’s recommendation was to use its full approved current and rely on the vehicle or charger’s scheduling controls when energy should be delivered later. That approach can place charging in a preferred rate window without extending the active session unnecessarily.

The ADAC results do not establish one optimum setting for every car, temperature and tariff. They do establish a consistent pattern across the five tested vehicles: every reduction in AC charging power increased losses, while the 11-kilowatt condition held losses to roughly 5% to 7% across the group.

By Thomas Caldwell — AMI’s senior editor for mechanical and mobility engineering, covering vehicle electronics, systems integration, electrification, chassis systems, propulsion, and safety policy.